Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. These low-speed pre-ignition events can occur even before spark is initiated in the cylinder. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. As such, if the pre-ignition is not mitigated in a timely manner, engine damage can occur. Accordingly, various strategies have been developed for early detection and mitigation of pre-ignition. The various approaches include, for example, cylinder fuel enrichment.
The inventors herein have recognized that a reduction in engine load alongside the various pre-ignition mitigating steps can help to rapidly reduce the risk of pre-ignition induced engine damage. Thus, in one example, pre-ignition can be mitigated by a method comprising: in response to an indication of pre-ignition, opening a scroll valve coupled to an outer scroll of a multi-scroll exhaust turbine to reduce engine load and reduce internal residuals by reducing exhaust manifold pressure. In this way, engine load can be substantially reduced and level of internal residuals can be quickly reduced lowering the likelihood of engine hardware damage.
In one example, in response to an indication of pre-ignition, such as based on the output of a knock sensor and/or a crankshaft acceleration sensor, a scroll valve coupled to an outer scroll of a multi-scroll exhaust turbine may be moved to a more open position. For example, the scroll valve may be immediately moved to a fully open position. Alternatively, the scroll valve may be moved to the fully open position incrementally, such as in discrete steps (e.g., of 5-10% increments). By opening the scroll valve, an exhaust manifold pressure can be rapidly reduced, thereby reducing an amount (as well as a temperature and pressure) of residuals trapped in the cylinder, lowering the in-cylinder conditions that contribute to pre-ignition. Initially, engine load (aircharge trapped in cylinder) may be increased due to reduction in exhaust manifold pressure but with more favorable in-cylinder conditions. With reduction in throttle angle and the resulting reduction in turbo speed, load (air charge trapped in cylinder) can be reduced. The scroll valve opening may be based on the indication of pre-ignition (e.g., based on the magnitude of the knock sensor output) and/or based on a turbine inlet temperature, with the scroll valve opening increased as the indication of pre-ignition increases or as the turbine inlet temperature increases above a threshold temperature. The scroll valve may then be maintained open until an exhaust manifold pressure is below a threshold pressure, or until the turbine inlet temperature is below the threshold temperature. Once the engine load and turbine inlet temperature have been sufficiently reduced, the scroll valve may be moved to a closed position based on turbine speed. Wastegate, throttle, charge motion control valve and EGR valve adjustments may be coordinated with the scroll valve adjustment to further reduce engine load in response to the indication of pre-ignition. Further still, the scroll valve adjustment may be coordinated alongside other pre-ignition mitigating steps, such as cylinder fuel enrichment, to enhance pre-ignition mitigation.
It will be appreciated that similar scroll valve adjustments may also be used in response to the engine approaching or being at, other engine hardware limits (e.g., in response to knock). In each case, when the limit is met or approached, scroll valve adjustments, by themselves or in coordination with wastegate, throttle and EGR valve adjustments, may be used to quickly improve in-cylinder conditions. In this way, scroll valve adjustments may be advantageously used to mitigate pre-ignition.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.